NAME

Moose::Cookbook::Extending::Recipe1 - Moose extension overview

DESCRIPTION

Moose provides several ways in which extensions can hook into Moose and change its behavior. Moose also has a lot of behavior that can be changed. This recipe will provide an overview of each extension method and give you some recommendations on what tools to use.

If you haven't yet read the recipes on metaclasses, go read those first. You can't write Moose extensions without understanding the metaclasses, and those recipes also demonstrate some basic extension mechanisms, such as metaclass subclasses and traits.

Playing Nice With Others

One of the goals of this overview is to help you build extensions that cooperate well with other extensions. This is especially important if you plan to release your extension to CPAN.

Moose comes with several modules that exist to help your write cooperative extensions. These are Moose::Exporter and Moose::Util::MetaRole. By using these two modules, you will ensure that your extension works with both the Moose core features and any other CPAN extension using those modules.

PARTS OF Moose YOU CAN EXTEND

The types of things you might want to do in Moose extensions fall into a few broad categories.

Metaclass Extensions

One way of extending Moose is by extending one or more Moose metaclasses. For example, in Moose::Cookbook::Meta::Recipe4 we saw a metaclass subclass that added a table attribute to the metaclass. If you were writing an ORM, this would be a logical extension.

Many of the Moose extensions on CPAN work by providing an attribute metaclass extension. For example, the MooseX::AttributeHelpers distribution provides a new attribute metaclass that lets you delegate behavior to a non-object attribute (a hashref or simple number).

A metaclass extension can be packaged as a subclass or a role/trait. If you can, we recommend using traits instead of subclasses, since it's much easier to combine disparate traits than it is to combine a bunch of subclasses.

Providing Sugar Functions

As part of a metaclass extension, you may also want to provide some sugar functions, just like Moose.pm does. Moose provides a helper module called Moose::Exporter that makes this much simpler. We will be use Moose::Exporter in several of the extension recipes.

Object Class Extensions

Another common Moose extension technique is to change the default object class's behavior. For example, the MooseX::Singleton extension changes the behavior of your objects so that they are singletons. The MooseX::StrictConstructor extension makes the constructor reject arguments which don't match its attributes.

The Moose::Util::MetaRole module lets you apply roles to the base object class, as well as the meta classes just mentioned.

Providing a Role

Some extensions come in the form of a role for you to consume. The MooseX::Object::Pluggable extension is a great example of this. In fact, despite the MooseX name, it does not actually change anything about Moose's behavior. Instead, it is just a role that an object which wants to be pluggable can consume.

If you are implementing this sort of extension, you don't need to do anything special. You simply create a role and document that it should be used via the normal with sugar:

package MyApp::User;
use Moose;
with 'MooseX::My::Role';

New Types

Another common Moose extension is a new type for the Moose type system. In this case, you simply create a type in your module. When people load your module, the type is created, and they can refer to it by name after that. The MooseX::Types::URI and MooseX::Types::DateTime distributions are two good examples of how this works. These both build on top of the MooseX::Types extension.

ROLES VS TRAITS VS SUBCLASSES

It is important to understand that roles and traits are the same thing. A trait is simply a role applied to a instance. The only thing that may distinguish the two is that a trait can be packaged in a way that lets Moose resolve a short name to a class name. In other words, with a trait, the caller can refer to it by a short name like "Big", and Moose will resolve it to a class like MooseX::Embiggen::Meta::Attribute::Role::Big.

Implementing an extension as a (set of) metaclass or base object role(s) will make your extension more cooperative. It is hard for an end-user to effectively combine together multiple metaclass subclasses, but it is very easy to combine roles.

USING YOUR EXTENSION

There are a number of ways in which an extension can be applied. In some cases you can provide multiple ways of consuming your extension.

Extensions as Metaclass Traits

If your extension is available as a trait, you can ask end users to simply specify it in a list of traits. Currently, this only works for (class) metaclass and attribute metaclass traits:

If your extension applies to any other metaclass, or the object base class, you cannot use the trait mechanism.

The benefit of the trait mechanism is that is very easy to see where a trait is applied in the code, and consumers have fine-grained control over what the trait applies to. This is especially true for attribute traits, where you can apply the trait to just one attribute in a class.

Extensions as Metaclass (and Base Object) Subclasses

Moose does not provide any simple APIs for consumers to use a subclass extension, except for attribute metaclasses. The attribute declaration options include a metaclass option a consumer of your extension can use to specify your subclass.

This is one reason why implementing an extension as a subclass can be a poor choice. However, you can force the use of certain subclasses at import time by calling Moose->init_meta for the caller, and providing an alternate metaclass or base object class.

If you do want to do this, you should look at using Moose::Exporter to re-export the Moose.pm sugar function. With Moose::Exporter, if your exporting class has an init_meta method, Moose::Exporter makes sure that this init_meta method gets called when your class is imported.

Then in your init_meta you can arrange for the caller to use your subclasses:

NOTE: Make sure that your init_meta returns the metaclass object, just as Moose->init_meta does.

Extensions as Metaclass (and Base Object) Roles

Implementing your extensions as metaclass roles makes your extensions easy to apply, and cooperative with other role-based extensions for metaclasses.

Just as with a subclass, you will probably want to package your extensions for consumption with a single module that uses Moose::Exporter. However, in this case, you will use Moose::Util::MetaRole to apply all of your roles. The advantage of using this module is that it preserves any subclassing or roles already applied to the user's metaclasses. This means that your extension is cooperative by default, and consumers of your extension can easily use it with other role-based extensions. Most uses of Moose::Util::MetaRole can be handled by Moose::Exporter directly; see the Moose::Exporter docs.

As you can see from this example, you can use Moose::Util::MetaRole to apply roles to any metaclass, as well as the base object class. If some other extension has already applied its own roles, they will be preserved when your extension applies its roles, and vice versa.

Providing Sugar

With Moose::Exporter, you can also export your own sugar functions, as well as those from other modules:

This can be combined with metaclass and base class roles quite easily.

LEGACY EXTENSION MECHANISMS

Before the existence of Moose::Exporter and Moose::Util::MetaRole, there were a number of other ways to extend Moose. In general, these methods were less cooperative, and only worked well with a single extension.